1. Technical Field
The present invention relates to an insulating film and a semiconductor device having a field-effect transistor made by using the same.
2. Related Art
In order to ensure the amount of an electrical charge induced in the channel of a MISFET (metal-insulator- semiconductor field-effect transistor), it has been usual practice to employ a gate insulating film having a small thickness and thereby increase the capacitance of the transistor. As a result, a reduction has been accelerated in the thickness of a SiO2 film used as the gate insulating film and its thickness is now nearly as low as much less than 1 nm.
However, the gate-leakage current of a SiO2 film is so large that the dissipation of standby energy makes power consumption incontrollable. For example, the gate-leakage current of a SiO2 film having a thickness of 0.8 nm is as much as 1 kA/cm2 and presents a serious problem in respect of power consumption.
A large film thickness is effective for low power consumption. Accordingly, studies have been made to use a substance having a high dielectric constant (high-K dielectric) to make an insulating film which can realize a large electrical charge, even if its thickness may be larger than that of a SiO2 film.
There are known metal oxides for stable insulating films having a high dielectric constant, including HfO2, ZrO2 and silicates thereof (HfSiO4 and ZrSiO4) as particularly promising materials.
However, it has been a drawback of those high-K dielectric metal oxides that they tend to include oxygen vacancies easily. The presence of oxygen vacancies makes it difficult to realize a reduction of leakage current, since a level thereby caused occurs in a band gap. Although no satisfactory solution has been found for those problems as yet, several proposals have been made.
A first proposal lies in the use of nitride such as HfON, ZrON, HfSiON or ZrSiON (reference is made to JP-A-2005-258854 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)).
A second proposal lies in the use of a ternary metal oxide, e.g., an aluminate such as HfAlO or ZrAlO (reference is made to JP-A-2005-311300).
There is also known a film of silicate (HfSiO4 or ZrSiO4) having a metal element concentration of 17 at % or less and containing 1 at % or more of Mg or the like (reference is made to JP-A-2003-289140).
As a result of our serious study, we, the inventors of the present invention, have discovered the following facts.
The problem of the leakage current of a film of HfO2, ZrO2 or the silicate thereof (HfSiO or ZrSiO) does not essentially reside in the oxygen vacancy itself, but resides in the lack of charge compensation for the oxygen vacancy and the mobility of the oxygen vacancy.
As a result of the free movement of the oxygen vacancy for which no charge compensation is made, the following problems occur:
(1) The mobile oxygen vacancy acts as a trigger to facilitate the precipitation of a crystal from the amorphous state and thereby makes it impossible to guarantee the uniform properties of the insulating film (problem of phase separation and crystallization);
(2) The oxygen vacancy for which no charge compensation is made causes a level to occur in a band gap in the vicinity of Ec and form a source for leakage current (problem of leakage current caused by the oxygen vacancy); and
(3) The movement of the oxygen vacancy gives rise to a structural defect affecting long-term reliability (problem of reliability).
None of those problems would occur if charge compensation were made for any oxygen vacancy in a film of HfO2, ZrO2 or a silicate thereof, and if the oxygen vacancy were fixed.
However, none of the proposals hitherto made has been able to solve any of the problems (1) to (3), as will be discussed below.
Referring to the first proposal relying on nitride, nitrogen has only a small power of fixing any oxygen vacancy and a large amount of nitrogen is required for suppressing crystallization. However, the use of a large amount of nitrogen brings about a seriously narrowed band gap. For example, the band offset ΔEc between the conduction band and the silicon substrate drops by nearly 1.0 eV and the band offset ΔEv between the valence band and the silicon substrate drops by about 0.5 to 1.5 eV. Accordingly, a seriously increased leakage current occurs, though the amorphous state may be maintained. Nitriding also presents a problem in respect of long-term reliability. The introduction of nitrogen usually brings about an increase in oxygen vacancy. However, as nitrogen cannot fix any oxygen vacancies satisfactorily, the movement of the oxygen vacancy produces a structural change in a long period of time. The structural change is accompanied by a fixed charge and fixed polarization and thereby brings about a serious worsening in dielectric properties.
As regards the second proposal relying on an aluminate film, aluminum has only a small power of fixing any oxygen vacancy and a large amount of aluminum is required for suppressing crystallization. The use of a large amount of aluminum brings about a serious drop of ΔEc and an increased leakage current. In order to fix any oxygen vacancy with a trivalent cation additive, it is generally necessary for two added elements and the oxygen vacancy to form a complex (for example, when Al is added, a complex expressed as “Al-Vo-Al”, where Vo stands for oxygen vacancy) , the formation of such a complex on the whole film requires a considerably large amount of heat (i.e. a high temperature for a long time). An incomplete complex, such as Al-Vo, is also formed and forms a new charge source. That is one of the reasons why aluminum has only a small power of fixing any oxygen vacancy. Another problem is that aluminum facilitates phase separation and crystallization, since Al2O3 is stable in terms of energy, and since Al has a very small ion radius as compared with Hf or Zr. As the introduction of aluminum results in a large amount of oxygen vacancies, mobile oxygen vacancies facilitate phase separation. Moreover, the use of a large amount of aluminum causes various levels to be produced in the band gap by oxygen vacancies and structural changes, thereby bringing about a serious worsening in properties of the insulating film.
Therefore, it has been difficult to reduce by employing those methods the leakage current of a film of HfO2, ZrO2 or a silicate thereof without impairing its original properties. It has been impossible to produce an insulating film which can fix any oxygen vacancy powerfully, while not causing any narrowing of the band gap.